An EEG instrument's common mode rejection ratio is ideally expected to be:

The common mode rejection ratio (CMRR) of an EEG instrument is a critical performance metric, indicating how well the device can eliminate common noise signals that are present on both input channels, such as electrical interference or artifacts. A very high CMRR means that the EEG system is effective at distinguishing between the actual brain activity (the differential signals) and the noise (the common mode signals), allowing for a clearer and more accurate representation of the brain's electrical activity.

In practical terms, a high CMRR reduces the potential for confounding factors that can lead to misinterpretation of the EEG data. This is particularly crucial in clinical settings, where accurate diagnosis and assessment rely on the quality of the recorded signals. A high CMRR prevents artifacts from contaminating readings, thereby enhancing the reliability of the EEG results.

Ultimately, achieving a high CMRR is essential for ensuring that the EEG instrument can effectively detect genuine neurophysiological signals amidst a backdrop of electrical noise and interference.